Does pseudoneglect influence pupillary light or dark response?

Pupillary light response (PLR) is modulated by the allocation of spatial attention. Larger pupil constrictions for bright stimuli presented on the left side are considered indicative of pseudoneglect, a subtle attentional bias observed in neurotypical populations. This study aimed to replicate this effect using the split-screen method-a newly introduced measure of spatial attentional bias-while accounting for factors such as contraction anisocoria by recording from both pupils. Additionally, we introduced conditions with and without competing stimuli (a black patch on the opposite side to the original white patch that is supposed to elicit pupil contraction) to investigate the role of visual competition in PLR modulation and explored the pupillary dark response (PDR) to assess whether attentional biases affect pupil dilation. Contrary to our hypothesis, we did not observe a significant pseudoneglect effect, as pupil constriction was not consistently greater for left-sided stimuli. We found clear evidence for contraction anisocoria, whereby ipsilateral stimuli produce stronger constrictions than contralateral stimuli, thus highlighting the need to account for this physiological effect in future studies. Regarding PDR, we did not find significant attentional modulation or evidence of dilation anisocoria as pupil dilation amplitudes were similar across both hemifields. These findings suggest that although the split-screen method may reveal physiological asymmetries like anisocoria, its sensitivity to attentional biases in neurotypical populations still requires further investigation.